I had the same thoughts regarding your solution/balance theory as well as the cost, but like I mentioned I'm also a newb & haven't started growing quite yet so I don't have a clue. I was thinking that a little more solution per plant would give you a bit of 'fudge' room if you were trying to maximize the change interval without depriving the plants, and without a TDS meter. I'm interested to see what the more experienced (and poor, like me) forum members think about that.
I also didn't have the $ for a meter, but then I found the Harbor Freight meter which I'm going to use. It's only $17 + shipping and from what I've been reading about EC & TDS I figure as long as the meter is temp compensated (it is), gives consistent readings (it does), and you 'calibrate' it to your water &
nutrients it should work reasonably well.
This link and
this one are two of the reasons I'm going to try it, even though it's not 'lab' quality by any stretch.